Plasma treatment of filter media

ABSTRACT

The present invention is drawn to an improved electrostatic filter medium comprising a web of electret fibers which have been treated with fluorine-containing plasmas at a deposition amount of about 0.03 g/m 2  to about 1.5 g/m 2 ; rinsing; and drying prior to being electrically charged. The present invention is further drawn to a method of producing non-woven webs which have been treated with a fluorine-containing plasma at a deposition amount of about 0.03 g/m 2  to about 1.5 g/m 2 ; rinsed; and dried prior to electrostatically charging the web.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the improvement of electrostaticfilter media by the use of a fluorine-containing plasma.

[0003] 2. Description of the Related Arts

[0004] Filters having an electrostatic charge are known in the art andhave a variety of uses. The electrostatic charge provides for increasedfilter efficiency against a variety of substances, especially chargedparticulates such as dust. Charged filters are commonly used today inhousehold heating and air conditioning filters and in vacuum bags.

[0005] The filters typically contain a non-woven web of electret fibers.These webs can be formed by a variety methods. For example, a thin filmor monolayer of polymer material can be extruded and charged via coronadischarge. The charged film is then fibrillated and the resultingfibrils formed into a non-woven web via needle punching or other knownmeans. In another method, a polymer is melt blown as a fine fiber andthen fashioned into a non-woven web. The fibers are charged either asthey exit the orifice of the extruder during melt blowing or after theyare assembled into a web. Charging can be carried out by coronaexposure, ion bombardment, etc.

[0006] The conventional electrostatic filters, however, suffer fromunstable charging. The electrostatic charge possessed by the fiberelectrets tends to dissipate over time. This presents a problem withrespect to storage stability. The consumer may be confronted withfluctuating filtering efficiency based on the duration betweenmanufacture and use.

[0007] Furthermore, the charge loss may be accelerated during use,thereby requiring a premature change of the filter. This acceleratedcharge loss during use is normally caused by the make up of thecomposition being filtered. One known problem in this regard is thepresence of oily aerosols. It is known that oil is especiallyproblematic with respect to a polyolefin based filter web such aspolypropylene.

[0008] One proposal for dealing with this problem is set forth in U.S.Pat. No. 5,411,576. Here, oily aerosol resistance is stated to beimproved by incorporating a fluorochemical additive into a polypropylenefiber electret. The fluorochemical is blended with the polypropylenebefore extrusion and is taught to be used in an amount of 0.2 to 10% byweight. The fluorochemical must be melt processable and preferably has amolecular weight in the range of about 500 to 2500.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a filterhaving improved electrostatic charge stability.

[0010] It is another object of the invention to provide a filter havinggood resistance against oils.

[0011] A further of object of the invention is to provide a washableelectrostatic media.

[0012] Another object of the present invention is to provide a processfor making electrets having improved charge stability.

[0013] A further object of the present invention is to provide a processfor making a filter having improved properties.

[0014] These and other objects of the invention are achieved by a filtermedium, comprising a web of electret fibers, said fibers having beenformed from a material that was treated with a fluorine-containingplasma and washed prior to being electrically charged.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventor has unexpectedly discovered that treatingthe web material with a fluorine-containing plasma to create adeposition of about 0.03 g/m² to about 1.5 g/m² of a fluoropolymer,followed by washing and drying, increases the charge stability of theelectret. In addition, the filter efficiency is also improved.

[0016] The plasma treatment can be carried out at virtually any stage ofthe filter manufacturing process, so long as it occurs prior to thecharging step. Preferably, in the melt blown fabrication process, theplasma treatment is carried out after web formation, although plasmatreatment could occur at any time after the fibers leave the orifice ofthe melt blow extruder. Correspondingly, the charging of the fibers ispreferably carried out on the non-woven web after the plasma treatment.

[0017] Similarly, in the fibrillating based fabrication process, theplasma treatment can occur at any time once the material leaves theorifice of the extruder; i.e., simultaneously with extrusion, so long asthe plasma treatment occurs before the charging step. However, forconvenience, the plasma treatment is preferably carried out on the thinfilm and prior to fibrillation. Plasma treatment after fibrillation isalso contemplated, as is plasma treatment after web formation. Chargingcan occur at any stage of web formation after the plasma treatment hasbeen carried out, including after needle punching or other binding step.

[0018] The plasma treatment uses a fluorine-containing plasma. Thismeans that the plasma contains a fluorine source such that a fluorinefree radical or ion is formed. The fluorine source can be elementalfluorine or a fluorine-containing compound. Examples of suitablefluorine sources include short chain fluorocarbons having 1 to 8 carbonatoms, preferably 1-3 carbon atoms, wherein at least one hydrogen atomhas been replaced with a fluorine atom. Preferably, at least 25 mol % ofthe hydrogen atoms have been replaced with fluorine atoms, morepreferably at least 50%. The fluorocarbons can be saturated orunsaturated. Other fluorine sources include fluorosilanes. Concreteexamples of fluorine sources include fluorine, trifluoromethane,tetrafluoroethane, and tetrafluorosilane (SiF₄).

[0019] The plasma is typically comprised of the fluorine source, only,although other materials can be present. In one embodiment, the fluorinesource is mixed with a carrier gas such as nitrogen, which may causehigher fluorine radical generation in the plasma.

[0020] Suitable plasma conditions to ensure deposition of about 0.03g/m² to about 1.5 g/m², preferably about 0.05 g/m² to 1.0 g/m², morepreferably about 0.07 g/m², of a fluoropolymer can be readily determinedby conventional means. The power, duration, and pressure can varysignificantly depending on the size and shape of the chamber and thecomposition of the plasma. In general the power ranges from 10 to 5000watts, the duration of the treatment is from one second to five minutesand the process pressure is from 10 milliTorr to 1000 milliTorr.Subsequent to plasma treatment the filter is washed an aqueous solventmixture, such as an isopropyl alcohol/water mixture, or water and dried.

[0021] The treated material, in film, fiber, or web form, iselectrically charged in order to form an electret. This charginggenerally is carried out by corona discharge although any of the methodsknown in the art for charging such materials can be used.

[0022] The fiber material used in the filter of the present inventioncan be any of the known materials, including polycarbonates,polyhalocarbons, polyesters, nylons and polyolefins. Of these,polyolefins such as polyethylene and polypropylene, are preferred withpolypropylene being most preferred.

[0023] The web formed can be woven or non-woven. Typically, the web isnon-woven and formed from fibers that resulted from either fibrillatinga thin film of polymer material or from melt blowing the fibersdirectly. Meltblown webs may be formed using standard meltblowntechnology as described in U.S. Pat. No. 2,411,660; U.S. Pat. No.3,849,241; U.S. Pat. No. 4,215,682; U.S. Pat. No. 2,466,906; U.S. Pat.No. 2,491,889; U.S. Pat. No. 2,740,184; U.S. Pat. No. 2,810,426; U.S.Pat. No. 3,003,304; and Wente, Va. Tech Rep. No. PB111437, Naval Rsch.Lab., NRL-4364, Apr. 15, 1954. Fibrillated webs may be formed accordingto the standard technology as described in U.S. Pat. No. 3,998,916 andU.S. Pat. No. 4,456,648. The size of the fibers is not particularlylimited in the present invention as all of the known and customary sizesand shapes may be employed herein.

[0024] The web can be used by itself as a filter or it can be combinedwith other layers. For example, the web can be laminated with a scrim ornetting, a woven layer such as fabric, or with a non-woven layer. Ofcourse, multiple webs and multiple layers are also contemplated.

EXAMPLE 1

[0025] Meltblown media made of 40 gm/m² polypropylene was treated with ahexafluoropropylene gas plasma having the following characteristics:Power: 3000 W Plasma type: RF Line speed: 3 meter/min. Gas level: 3L/min. Deposition 0.25 gm/m² rate:

[0026] After plasma treatment the media was rinsed in water and theefficiency of the media was tested as follows.

[0027] The efficiency of the filter media was tested using a challengeaerosol of KCl. The aerosol was generated using a nebulizer and thenneutralized. Using a laser particle counter the number of particlesbetween 0.03 μm and 0.5 μm in size were measured before and after thefilter. The ratio of the number of particles before the filter to thenumber of particles after the filter is the efficiency of the filter.The washed filter had the following properties, measured as describedabove. Efficiency: 87% (0.3-5 μm particle size) Pressure drop: 12.5mmH₂O Test Flow rate: 30 CFM Test aerosol: KCl Filter size: 10 cm × 10cm × 0.5 cm, 5 pleats/inch

[0028] The filter was then loaded with 2 gm of SAE fine dust to simulatean in use application. The filter was rinsed with water and theefficiency tested again at the above conditions, upon which it showedthe following properties. Efficiency: 87.8% Pressure Drop: 12.5 mmH₂O

EXAMPLE 2

[0029] A meltblown polypropylene web was plasma treated at theabove-described conditions but with a deposition of 0.05 gm/m² (Type 1);0.025 gm/m² (Type 2); no plasma treatment (Type 3). Each of the webswere corona charged at 15 kV. The webs had a base weight of 50 gm/m².The efficiency of the webs was tested before and after washing withwater. (“% Penetration”=100−% efficiency) Washed webs were dried at 40°C. Efficiency was tested using a polydisperse NaCl aerosol, 10 cm/secface velocity and is inverse of the percentage of penetration. TABLE 1Percent penetration sample type Type 1 Type 2 Type 3 before wash 10.2522.6 40.0 after wash, 1.2 40.6 41.0 40° dry

[0030] As shown in Table 1, web of the present invention hassynergistically improved properties.

EXAMPLE 3

[0031] A meltblown polypropylene web with a base weight of 15 gm/m² wasplasma treated with a deposition rate of 0.25g/m² and corona charged at15 kV. The media was tested before and after washing. The washed webswere dried at 40° C. The webs had the following properties before andafter washing.

[0032] Before wash: 90% penetration(particle size 0.3-0.5 μm)

[0033] After wash: 73% penetration

[0034] The pressure drop before and after the rinse step was same.

[0035] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

I claim:
 1. A filter medium, comprising a web of electret fibers, saidfibers having been formed from a material that was 1) treated with afluorine-containing plasma at a deposition amount of about 0.03 g/m² toabout 1.5 g/m²; 2) electrostatically charged 3) rinsed; and 4) dried 2.The filter medium according to claim 1, wherein said fibers are made ofa polyolefin.
 3. The filter medium according to claim 1, wherein saidfibers are made of polypropylene.
 4. The filter medium according toclaim 1, wherein said fluorine source is trifluoromethane,tetrafluoroethane, or tetrafluorosilane.
 5. The filter medium accordingto claim 1, wherein said web is non-woven.
 6. A method of forming anon-woven web, which comprises: (1) melt blowing polyolefin fibers intoa non-woven web; (2) treating said non-woven web with afluorine-containing plasma at a deposition amount of about 0.03 g/m² toabout 1.5 g/m²; (3) charging said treated non-woven web to form anon-woven web of electret fibers; (4) rinsing said treated non-wovenweb; and (5) drying said non-woven web.
 7. The method according to claim6, wherein said polyolefin fibers are made of polypropylene.
 8. Themethod according to claim 6, wherein said plasma contains atrifluoromethane, tetrafluoroethane, or tetrafluorosilane.
 9. A methodof forming a non-woven web, which comprises: (1) treating a thin film ofpolymer with a fluorine-containing plasma at a deposition amount ofabout 0.03 g/m² to about 1.5 g/m²; (2) rinsing said treated non-wovenweb; (3) drying said non-woven web; (4) electrically charging saidtreated thin film; (5) fibrillating said charged film to form fibers;and (6) forming said fibers into a non-woven web of electret fibers. 10.The method according to claim 9, wherein said polymer is polypropylene.11. The method according to claim 9, wherein said plasma contains atrifluoromethane, tetrafluoroethane, or tetrafluorosilane.
 12. In amethod of forming a web of electret fibers, which comprises chargingelectrically charging a filter material in film or fiber form, theimprovement which comprises treating the filter material with afluorine-containing plasma at a deposition amount of about 0.03 g/m² toabout 1.5 g/m²; rinsing the filter material and drying the filtermaterial prior to carrying out electrical charging of the filtermaterial.
 13. The filter medium of claim 1, wherein the treatment with afluorine-containing plasma is at a deposition amount of about 0.07 g/m².14. The method of claim 6, wherein the treatment with afluorine-containing plasma is at a deposition amount of about 0.07 g/m².15. The method of claim 9, wherein the treatment with afluorine-containing plasma is at a deposition amount of about 0.07 g/m².16. The method of claim 12, wherein the treatment with afluorine-containing plasma is at a deposition amount of about 0.07 g/m².